Currently, Orthogonal Frequency Division Multiplexing (OFDM) is a widely used transmission scheme that has been adopted for digital terrestrial broadcasting and a variety of other digital communications, such as IEEE 802.11a. In the OFDM method, a plurality of narrow band digital modulated signals are frequency multiplexed using a plurality of orthogonal subcarriers. OFDM is therefore an excellent transmission scheme for efficiently using frequencies.
Furthermore, in the OFDM method, one symbol duration is composed of a useful symbol duration and a guard interval duration. To provide periodicity within a symbol, a signal for a portion of the useful symbol duration is copied and inserted into the guard interval duration. This allows for elimination of the effect of interference between symbols produced by multipath interference. OFDM is therefore also highly resistant to multipath interference.
In recent years, analog television broadcasting has ceased in countries around the world, and efforts towards frequency reallocation are gaining momentum. In Europe, in addition to Standard Definition (SD) broadcasting for Digital Video Broadcasting-Terrestrial (DVB-T), a demand for High Definition (HD) service is rising. Given these circumstances, progress has been made in the standardization of DVB-T2, the second generation of European digital terrestrial broadcasting. In the DVB-T2 system, as shown in FIG. 34, DVB-T2 frames are used. A DVB-T2 frame is composed of a P1 symbol, P2 symbols, and data symbols.
First, P1 symbols are described.
A P1 symbol is set to have a Fast Fourier Transform (FFT) size of 1 k (1024). As shown in FIG. 35, a guard interval duration is provided on both sides of the useful symbol duration. Note that FIG. 35 shows a P1 symbol in the time domain. The guard intervals in a P1 symbol differ from the guard interval in the conventional Integrated Services Digital Broadcasting-Terrestrial (ISDB-T) and DVB-T standards. In a P1 symbol, a signal for 59 μs from the earlier half of the useful symbol duration is copied and inserted into the guard interval duration that is earlier than the useful symbol duration (hereinafter referred to as “earlier guard interval duration”). A signal for 53 μs from the later half of the useful symbol duration is copied and inserted into the guard interval duration that is after the useful symbol duration (hereinafter referred to as “later guard interval duration”). Furthermore, when copying and inserting, the original signal is frequency shifted by a predetermined fSH amount before insertion into the guard interval duration (the earlier guard interval duration or the later guard interval duration). In this case, fSH corresponds to one physical subcarrier interval of a P1 symbol. In other words, the signal for the earlier guard interval duration and the signal for the later guard interval duration are one P1 symbol subcarrier higher in frequency than the signal for the useful symbol duration. Note that as shown in FIG. 35, the entire useful symbol is used in the guard intervals in a P1 symbol.
As shown in FIG. 36, a P1 symbol is composed of active carriers and null carriers (unused carriers). Note that FIG. 36 shows a P1 symbol in the frequency domain.
A P1 symbol includes information (hereinafter referred to as “P1 transmission information”) such as the following: information on whether the transmission format of the P2 symbols and the data symbols is Multiple-Input-Single-Output (MISO) or Single-Input-Single-Output (SISO) (hereinafter referred to as “MISO/SISO information”), information on what the FFT size of the P2 symbols and data symbols is (hereinafter referred to as “FFT size information”), information on whether Future Extension Frames (FEFs) are included (hereinafter referred to as “FEF inclusion information”), and the like. In this context, an FEF is a period for future service transmission differing from DVB-T2. An FEF is inserted between DVB-T2 frames, and a P1 symbol is located at the top of an FEF as well.
The following describes generation of a P1 symbol.
FIG. 37 shows the structure of a P1 symbol generation unit 1000 that generates a P1 symbol. The P1 symbol generation unit 1000 is provided with a sequence transformation unit 1001, a differential modulation unit 1002, a scrambling unit 1003, a CDS table generation unit 1004, a padding unit 1005, an IFFT unit 1006, and a GI adding unit 1007.
As described above, P1 transmission information is transmitted by a P1 symbol. This information is represented as a three-bit S1 signal and a four-bit S2 signal. The three-bit S1 signal and the four-bit S2 signal are input into the sequence transformation unit 1001. The sequence transformation unit 1001 stores a transform table such as the one shown in FIG. 38. By referring to the table, the sequence transformation unit 1001 transforms the three-bit S1 signal into a 64-bit sequence CSSS1, represented by Equation 1 below, and the four-bit S2 signal into a 256-bit sequence CSSS2, represented by Equation 2 below. The “Value” column in FIG. 38, represents the value that is input into the sequence transformation unit 1001, whereas the “Sequences (hexadecimal) CSSS1 and CSSS2” represent the sequences after transformation (the sequences output from the sequence transformation unit 1001). Note that in FIG. 38, for the sake of convenience, the transformed sequences CSSS1 and CSSS2 are represented in hexadecimal.CSSS1=(CSSS1,0, . . . , CSSS1,63)  Equation 1CSSS2=(CSSS2,0, . . . , CSSS2,255)  Equation 2
The sequence transformation unit 1001 uses the sequence CSSS1 represented by Equation 1 and the sequence CSSS2 represented by Equation 2 to construct the 384-bit signal sequence MSS_SEQ shown in Equation 3 below, outputting the signal sequence MSS_SEQ to the differential modulation unit 1002. Note that the signal sequence MSS_SEQ includes two identical S1 signals.
                                                        MSS_SEQ              =                            ⁢                              (                                                      MSS_SEQ                    0                                    ,                  …                  ⁢                                                                          ,                                      MSS_SEQ                    383                                                  )                                                                                        =                            ⁢                              (                                                      CSS                                          S                      ⁢                                                                                          ⁢                      1                                                        ,                                      CSS                                          S                      ⁢                                                                                          ⁢                      2                                                        ,                                      CSS                                          S                      ⁢                                                                                          ⁢                      1                                                                      )                                                                                        =                            ⁢                              (                                                      CSS                                                                  S                        ⁢                                                                                                  ⁢                        1                                            ,                      0                                                        ,                  …                  ⁢                                                                          ,                                      CSS                                                                  S                        ⁢                                                                                                  ⁢                        1                                            ,                      63                                                        ,                                      CSS                                                                  S                        ⁢                                                                                                  ⁢                        2                                            ,                      0                                                        ,                  …                  ⁢                                                                          ,                                                                                                                      ⁢                                                CSS                                                            S                      ⁢                                                                                          ⁢                      2                                        ,                    255                                                  ,                                  CSS                                                            S                      ⁢                                                                                          ⁢                      1                                        ,                    0                                                  ,                …                ⁢                                                                  ,                                  CSS                                                            S                      ⁢                                                                                          ⁢                      1                                        ,                    63                                                              )                                                          Equatio        ⁢                                  ⁢        n        ⁢                                  ⁢        3            
The differential modulation unit 1002 performs the differential modulation shown in Equation 4 below on the signal sequence MSS_SEQ input from the sequence transformation unit 1001 and outputs a signal sequence MSS_DIFF resulting from differential modulation to the scrambling unit 1003. The differential modulation performed by the differential modulation unit 1002 is Differential Binary Phase Shift Keying (DBPSK).MSS_DIFF=DBPSK(MSS_SEQ)  Equation 4
Specifically, the differential modulation unit 1002 treats a reference signal MSS_DIFF−1 as 1, as shown in Equation 5 below, and performs differential modulation based on Equation 6 below on the signals MSS_SEQi (i=0, 1, . . . , 383) constituting the signal sequence MSS_SEQ input from the sequence transformation unit 1001. The differential modulation unit 1002 outputs the differentially modulated signal MSS_DIFFi to the scrambling unit 1003.
                              MSS_DIFF                      -            1                          =        1                            Equation        ⁢                                  ⁢        5                                          MSS_DIFF          i                =                  {                                                                      MSS_DIFF                                      i                    -                    1                                                                                                                    :                                          MSS_SEQ                      i                                                        =                  0                                                                                                      -                                      MSS_DIFF                                          i                      -                      1                                                                                                                                        :                                          MSS_SEQ                      i                                                        =                  1                                                                                        Equation        ⁢                                  ⁢        6            
The scrambling unit 1003 performs the scrambling shown in Equation 7 below on the differentially modulated signal sequence MSS_DIFF from the differential modulation unit 1002 and outputs a scrambled signal sequence MSS_SCR to the padding unit 1005.MSS_SCR=SCRAMBLING(MSS_DIFF)  Equation 7
Specifically, the scrambling unit 1003 uses a signal PRBSi (i=0, 1, . . . , 383) based on a Pseudo Random Binary Sequence (PRBS) to scramble the differentially modulated signal MSS_DIFFi using Equation 8 below. The scrambling unit 1003 outputs a scrambled signal MSS_SCRi to the padding unit 1005.
                              MSS_SCR          i                =                              MSS_DIFF            i                    ×          2          ⁢                      (                                          1                2                            -                              PRBS                i                                      )                                              Equation        ⁢                                  ⁢        8            
The CDS table generation unit 1004 generates the Carrier Distribution Sequence (CDS) table shown in FIG. 39 to indicate the position k(i) (i=0, 1, . . . , 383) of each active carrier in a P1 symbol. Note that as shown in FIG. 39, identical S1 signals are transmitted in two positions in one P1 symbol, one in a high frequency domain and another in a low frequency domain, whereas an S2 signal is transmitted in a central frequency domain.
The padding unit 1005 treats the subcarriers at subcarrier positions k(i), as shown in the CDS table of the CDS table generation unit 1004 (see FIG. 39), as active carriers and maps the scrambled signal MSS_SCRi onto the subcarriers at subcarrier positions k(i). The padding unit 1005 then outputs the result to the IFFT unit 1006. Furthermore, the padding unit 1005 outputs the subcarriers at subcarrier positions not listed in FIG. 39 to the IFFT unit 1006 as null carriers.
The IFFT unit 1006 performs an Inverse Fast Fourier Transform (IFFT) with an FFT size of 1 k on the signal output by the padding unit 1005. The IFFT unit 1006 then outputs the result of the IFFT (a signal in the time domain of the useful symbol duration in FIG. 35) to the GI adding unit 1007.
The GI adding unit 1007 uses the signal in the useful symbol duration input from the IFFT unit 1006 to shift the frequency of an earlier portion of the signal in the useful symbol duration by fSH and insert the result in the earlier guard interval duration, and also to shift the frequency of a later portion of the signal in the useful symbol duration by fSH and insert the result in the later guard interval duration (see FIG. 35). A P1 symbol is thus generated.
Next, P2 symbols and data symbols are described.
A shared FFT size and guard interval ratio (ratio of the time of the guard interval duration to the time of the useful symbol duration) are used in P2 symbols and data symbols. As in DVB-T and ISDB-T, the guard interval duration in P2 symbols and in data symbols is provided before the useful symbol duration. A signal for a later portion of the useful symbol duration is copied and inserted into the guard interval duration provided before the useful symbol duration.
FIG. 40 shows combinations of the FFT size and guard interval ratio used in DVB-T2, as well as pilot patterns that can be set for these combinations. There are eight types of pilot patterns, from PP1 through PP8. In FIG. 40, “N/A” indicates a combination of FFT size and guard interval ratio not supported by standards.
Pilots with equal intervals (hereinafter referred to as “P2 pilots”) are inserted in P2 symbols. With an FFT size of 32 k and in SISO mode, a P2 pilot exists every six subcarriers. Otherwise, a P2 pilot exists every three subcarriers.
A P2 symbol includes any transmission parameter information necessary for reception (hereinafter referred to as “P2 transmission information”) such as the following: information on what the pilot pattern of the data symbols is (hereinafter referred to as “pilot pattern information”), information on whether the carrier extension mode is extended mode or normal mode (hereinafter referred to as “transmission mode information”), the number of symbols per frame, the modulation method, the coding ratio of the Forward Error Correction (FEC) code, and the like. Note that the number of P2 symbols is set in accordance with the FFT size of the P2 symbols, as shown in FIG. 41.
Technology for demodulating P1 symbols with the above DVB-T2 transmission format includes the method described in Non-Patent Literature 1.
FIG. 42 shows the structure of a P1 demodulation unit 2000 that demodulates P1 symbols. The P1 demodulation unit 2000 includes a P1 position detection unit 2001, a P1 narrow band fc error detection and correction unit 2002, an FFT unit 2003, a CDS table generation unit 2004, a P1 wide band fc error detection and correction unit 2005, and a P1 decoding unit 2006.
The P1 position detection unit 2001 uses an input signal to calculate the correlation (guard correlation) between the signal for the guard interval duration of a P1 symbol (earlier guard interval duration and later guard interval duration) and the signal for a predetermined section of the useful symbol duration of the P1 symbol. The P1 position detection unit 2001 calculates the interval integral of the calculated correlation over the time of each guard interval duration (earlier guard interval duration, later guard interval duration) and detects the position of the P1 symbol in the input signal by detecting the peak of the interval integral.
The calculation of the correlation takes into consideration the frequency shift of fSH that is added at the transmitting end. The above “predetermined section” is the earlier portion within the useful symbol duration for the earlier guard interval duration and is the later portion within the useful symbol duration for the later guard interval duration (see FIG. 35). The same is true for the calculation of the correlation by the P1 narrow band fc error detection and correction unit 2002, described below. The P1 narrow band fc error detection and correction unit 2002 calculates the correlation (guard correlation) between the signal for the guard interval duration of a P1 symbol (earlier guard interval duration and later guard interval duration) and the signal for a predetermined section of the useful symbol duration of the P1 symbol. Based on the correlation, the P1 narrow band fc error detection and correction unit 2002 detects the frequency error amount (narrow band carrier frequency error amount) that is equal to or less than the subcarrier interval of the P1 symbol. Based on the detected narrow band carrier frequency error amount, the P1 narrow band fc error detection and correction unit 2002 corrects a shift for the narrow band carrier frequency of the P1 symbol and outputs the P1 symbol whose shift for the narrow band carrier frequency has been corrected to the FFT unit 2003.
The FFT unit 2003 performs an FFT with an FFT size of 1 k on the signal in the time domain of the useful symbol duration of the P1 symbol, outputting the results of the FFT (a signal in the frequency domain of the useful symbol duration of the P1 symbol) to the P1 wide band fc error detection and correction unit 2005.
The CDS table generation unit 2004 generates a sequence showing the positions of active carriers (hereinafter referred to as “active carrier arrangement sequence”) and outputs the generated active carrier arrangement sequence to the P1 wide band fc error detection and correction unit 2005. The active carrier arrangement sequence is a sequence with a “1” at positions of active carriers, as shown in FIG. 39, and a “0” at other positions to indicate null carriers.
The P1 wide band fc error detection and correction unit 2005 uses the active carrier arrangement sequence input from the CDS table generation unit 2004 to detect the frequency error amount (wide band carrier frequency error amount) in units of subcarrier intervals of the P1 symbol in the signal output by the FFT unit 2003. Based on the detected wide band carrier frequency error amount, the P1 wide band fc error detection and correction unit 2005 corrects a shift for the wide band carrier frequency of the P1 symbol and outputs the active carriers in the P1 symbol whose shift for the wide band carrier frequency has been corrected to the P1 decoding unit 2006.
The following describes detection of the wide band carrier frequency error amount of the P1 symbol. As described above, subcarriers composing a P1 symbol are either active carriers or null carriers. Based on this fact, the power of each subcarrier is calculated, and the correction between the results of calculation and a known active carrier arrangement sequence (input from the CDS table generation unit 2004) is calculated while shifting the results of calculation one subcarrier at a time.
Since signals on which DBPSK has been performed are mapped to active carriers, the correlation for a shift amount at which the wide band carrier frequency error amount is zero is the sum of the power of all active carriers. This correlation is a larger value than the correlations for other shift amounts that include null carriers. Based on this fact, the shift amount yielding the largest correlation is the wide band carrier frequency error amount. It is thus possible to detect the wide band carrier frequency error amount. Note that the shift amount when there is no wide band carrier frequency error in the input signal is treated as a reference (shift amount “0”) here and in the following description.
The P1 decoding unit 2006 in FIG. 42 decodes the P1 symbol based on the active carriers in the P1 symbol input from the P1 wide band fc error detection and correction unit 2005 and extracts the P1 transmission information.
The P1 decoding unit 2006 is described with reference to FIG. 43. FIG. 43 shows the structure of the P1 decoding unit 2006 in FIG. 42. The P1 decoding unit 2006 is provided with a descrambling unit 2101, a differential demodulation unit 2102, and a pattern matching unit 2103. Note that here, a P1 symbol is decoded using only the S1 signal in the low frequency domain of the P1 symbol.
A signal sequence Act of active carriers is input from the P1 wide band fc error detection and correction unit 2005 in FIG. 42 into the descrambling unit 2101. The descrambling unit 2101 performs the descrambling shown in Equation 9 below on the signal sequence Act of active carriers and outputs a descrambled signal sequence DESCR to the differential demodulation unit 2102.DESCR=DESCRAMBLING(Act)  Equation 9
Specifically, the descrambling unit 2101 uses a signal PRBSi (i=0, 1, 2, . . . , 319), based on PRBS and used for multiplication at the transmitting end, to perform the descrambling shown in Equation 10 below on a signal Acti of active carriers, outputting a descrambled signal DESCRi to the differential demodulation unit 2102.
                              DESCR          i                =                              Act            i                    ×          2          ⁢                      (                                          1                2                            -                              PRBS                i                                      )                                              Equation        ⁢                                  ⁢        10            
A signal DESCRi (i=0, 1, . . . , 319) is input into the differential demodulation unit 2102 from the descrambling unit 2101. The differential demodulation unit 2102 performs differential detection by complex multiplication of a signal DESCRi (i=1, 2, . . . , 319) and a signal DESCR*i-1, which is the complex conjugate of a signal DESCRi-1 shifted by one active carrier. Note that the “*” suffix in superscript represents a complex conjugate (the same being true below as well). Based on the polarity of the real axis, the differential demodulation unit 2102 demodulates (hard decision) the signal DESCRi, and outputs a demodulated signal DEMODi to the pattern matching unit 2103. Processing by the differential demodulation unit 2102 is represented by Equation 11 below. The differential demodulation by the differential demodulation unit 2102 corresponds to DBPSK.
                              DEMOD          i                =                  {                                                                                          0                    ⁢                                          :                                        ⁢                                                                                  ⁢                                          real                      ⁡                                              (                                                                              DESCR                            i                                                    ·                                                      DESCR                                                          i                              -                              1                                                        *                                                                          )                                                                              ≥                  0                                                                                                                          1                    ⁢                                          :                                        ⁢                                                                                  ⁢                                          real                      ⁡                                              (                                                                              DESCR                            i                                                    ·                                                      DESCR                                                          i                              -                              1                                                        *                                                                          )                                                                              <                  0                                                                                        Equation        ⁢                                  ⁢        11            
Since i=0 is a reference, the differential demodulation unit 2102 performs demodulation (hard decision) based on the polarity of the real axis of the signal DESCR0, outputting a demodulated signal DEMOD0 to the pattern matching unit 2103.
The pattern matching unit 2103 divides the signals DEMOD0, DEMOD1, . . . , DEMOD319 differentially demodulated by the differential demodulation unit 2102 into a signal sequence DEMOD_CSSS1 (corresponding to the S1 signal) and a signal sequence DEMOD_CSSS2 (corresponding to the S2 signal), as shown in Equations 12 and 13 below.
                                                                        DEMOD_CSS                                  S                  ⁢                                                                          ⁢                  1                                            =                            ⁢                              (                                                      DEMOD                    0                                    ,                  …                  ⁢                                                                          ,                                      DEMOD                    63                                                  )                                                                                        =                            ⁢                              (                                                      DEMOD_CSS                                                                  S                        ⁢                                                                                                  ⁢                        1                                            ,                      0                                                        ,                  …                  ⁢                                                                          ,                                                                                                                                        ⁢                                  DEMOD_CSS                                                            S                      ⁢                                                                                          ⁢                      1                                        ,                    63                                                  )                            ⁢                                                                                                      Equation        ⁢                                  ⁢        12                                                                                    DEMOD_CSS                                  S                  ⁢                                                                          ⁢                  2                                            =                            ⁢                              (                                                      DEMOD                    64                                    ,                  …                  ⁢                                                                          ,                                      DEMOD                    319                                                  )                                                                                        =                            ⁢                              (                                                      DEMOD_CSS                                                                  S                        ⁢                                                                                                  ⁢                        2                                            ,                      0                                                        ,                  …                  ⁢                                                                          ,                                                                                                                      ⁢                              DEMOD_CSS                                                      S                    ⁢                                                                                  ⁢                    2                                    ,                  255                                            )                                                          Equation        ⁢                                  ⁢        13            
To calculate which of the sequences CSSS1, k (k=0, 1, . . . , 7) shown in FIG. 38 is the most probable, and to calculate which of the sequences CSSS2, k (k=0, 1, . . . , 15) shown in FIG. 38 is the most probable, the pattern matching unit 2103 performs the following processing. In this context, the index k is used to differentiate the eight sequences CSSS1 shown in FIG. 38 and to differentiate the 16 sequences CSSS2 shown in FIG. 38 (the same being true below as well).
The pattern matching unit 2103 calculates correlations CORRS1, k between each sequence CSSS1, k in FIG. 38 and the sequence DEMOD_CSSS1, as shown in Equation 14 below. The pattern matching unit 2103 also calculates correlations CORRS2, k between each sequence CSSS2, k in FIG. 38 and the sequence DEMOD_CSSS2, as shown in Equation 15 below.
                              CORR                                    S              ⁢                                                          ⁢              1                        ,            k                          =                              ∑                          i              =              0                        63                    ⁢                                    DEMOD_CSS                                                S                  ⁢                                                                          ⁢                  1                                ,                i                                      ⊕                          CSS                                                S                  ⁢                                                                          ⁢                  1                                ,                k                ,                i                                                                        Equation        ⁢                                  ⁢        14                            ⊕ indicates exclusive or        
                              CORR                                    S              ⁢                                                          ⁢              2                        ,            k                          =                              ∑                          i              =              0                        255                    ⁢                                    DEMOD_CSS                                                S                  ⁢                                                                          ⁢                  2                                ,                i                                      ⊕                          CSS                                                S                  ⁢                                                                          ⁢                  2                                ,                k                ,                i                                                                        Equation        ⁢                                  ⁢        15                            β indicates exclusive or        
The pattern matching unit 2103 estimates that the three-bit S1 signal (see FIG. 38) corresponding to the sequence CSSS1, k with the largest correlation among the eight correlations calculated using Equation 14 is the transmitted S1 signal. The pattern matching unit 2103 also estimates that the four-bit S2 signal (see FIG. 38) corresponding to the sequence CSSS2, k with the largest correlation among the 16 correlations calculated using Equation 15 is the transmitted S2 signal. The pattern matching unit 2103 acquires the P1 transmission information using the estimated S1 signal and S2 signal.